Displacement measurement technology finds wide applications in fields such as medical sector, structural integrity monitoring, and aeronautics. Ultraprecise displacement sensing (<10 μm) may also be extensively applied in micro- and nano-fabrication applications and industries, enabling high precision fabrication processes.
Fiber displacement sensors, such as Fabry-Perot interferometric sensor, fiber Bragg grating, and long period fiber gratings have been proposed before. The use of fiber Bragg grating based sensors for displacement measurement suffers from design fabrication complexities. The Fabry-Perot sensor uses liquids and flexible membrane and is prone to high stress. These Fiber displacement sensors are also in contact with the sample during the use. In one example, a fiber ring is placed in contact with the sample. When the sample is moved, the fiber ring will be mechanically deformed, thereby perturbing the optical environment of the waveguide along the fiber ring, resulting in changes of the output intensity or spectrum.
Existing products to monitor displacements with nanometer precision in a non-contact fashion includes capacitance probe. Generally, in order to apply the principle of the capacitor to detect the displacement and the position of a measurement object, it is necessary to have an electrode plate on the measurement object. When the object is non-metal, a metal plate needs to be attached to enable the measurement, which is inconvenient and undesirable for positioning and nano-fabrication applications. In addition, the reading obtained by the capacitor sensor may be affected, if the sample of the capacitor sensor during measurement is in an unfavorable electric or magnetic environment. As such, the capacitor sensor is generally not suitable for being applied to a sub-nanometer scale displacement measurement where the object is usually small and light weight, where the space for installation of the electrode plate is limited or where high noise signals caused by electromagnetic radiation exist.
For example, the resolution of conventional optical lithography is limited by the diffraction. Near-field optical lithography is the most promising emerging technique to break the diffraction limit. Generally, the distance between the writing head and the photo-resist (few micrometers generally) need to be controlled precisely, in the 100 s nm range, in order to ensure satisfying results. However, it is difficult to monitor the distance between the detector and the object with products such as capacitance probes, due to the size and material issues.
Another example is the measurement of parallelism of two surfaces. Parallelism is a critical condition to achieve for many engineering applications, e.g., fabrication or instrument installation. Although measuring the parallelism of two metal surfaces can be achieved by using three capacitance probes, it cannot be easily done on other surfaces such as semiconductors or polymers.
Therefore, there is a need for method and a system for measuring a displacement of an object which overcome the above described shortcomings.